Targeted deletion of matrix metalloproteinase-9 attenuates left ventricular enlargement and collagen accumulation after experimental myocardial infarction

Abstract

Matrix metalloproteinase-9 (MMP-9) is prominently overexpressed after myocardial infarction (MI). We tested the hypothesis that mice with targeted deletion of MMP9 have less left ventricular (LV) dilation after experimental MI than do sibling wild-type (WT) mice. Animals that survived ligation of the left coronary artery underwent echocardiographic studies after MI; all analyses were performed without knowledge of mouse genotype. By day 8, MMP9 knockout (KO) mice had significantly smaller increases in end-diastolic and end-systolic ventricular dimensions at both midpapillary and apical levels, compared with infarcted WT mice; these differences persisted at 15 days after MI. MMP-9 KO mice had less collagen accumulation in the infarcted area than did WT mice, and they showed enhanced expression of MMP-2, MMP-13, and TIMP-1 and a reduced number of macrophages. We conclude that targeted deletion of the MMP9 gene attenuates LV dilation after experimental MI in mice. The decrease in collagen accumulation and the enhanced expression of other MMPs suggest that MMP-9 plays a prominent role in extracellular matrix remodeling after MI.

Figure 1

Design of the protocol, with timing of experimental MI, echocardiographic studies, and sacrifice. LCA, left coronary artery. KO, MMP-9 KO animals; WT, congenic MMP-9 WT animals.

Figure 2

(a) Representative examples of staining with Masson’s trichrome and picrosirius red under polarized light of the infarcted region, 15 days after experimental MI (WT is represented by the left panels, MMP-9 KO by the right panels). Picrosirius red staining demonstrated the characteristic bright yellow of collagen fibrils under polarized light, with less deposition of picrosirius red–stained collagen in MMP-9 KO samples than in WT samples (scale bar: 200 μm). (b) Collagen volume fraction from picrosirius red–stained myocardium as percentage of stained tissue in muscle areas and connective tissue in the visual field of the section. (c) Collagen content of apical myocardium after MI in infarct size matching WT and KO animals. ^AP < 0.05.

Figure 3

(a) Representative examples of staining of α-actin (×400) and macrophages (Mac3, ×100; scale bar: 200 μm) of the infarcted region 15 days after MI, and quantitative fractional areas of (b) macrophages and (c) neutrophils from day 1 to day 7 after MI. ^AP < 0.05. (d) α-actin fractional area at day 15.

Figure 4

Western analysis for MMP-2, MMP-3, MMP-13, and TIMP-1 in animals that were not operated on (left panel) or 15 days after MI (right panel). MMP-9 KO mice had increased expression of these MMPs and TIMP-1 compared with WT mice.